1. Field of the Invention
The invention relates in general to integrated circuits and more particularly to the distribution of clock signals in programmable logic devices.
2. Description of the Related Art
A programmable logic device (PLD) integrated circuit comprises an array of functional blocks along with an interconnection network. The specific function of each block and the connections between blocks can be programmed by the user. For example, a typical PLD generally includes a control store in the form of a static random access memory array (SRAM), for instance, and includes functional units and interconnect wires that are programmed by writing to the control store to establish specific logic gates, storage elements and interconnect paths.
As PLDs increase in logic density, clock distribution delay becomes an increasingly significant fraction of the clock period. This clock distribution delay can impact device performance by degrading setup, hold and clock-to-output parameters, for example. Moreover, race conditions between clocks and data can result in race-through problems if proper clock planning techniques are not applied. A typical PLD often provides a variety of alternative clock sources. A PLD register, for instance, may be programmable to receive a clock signal from any of multiple different clock sources. Clock tree networks have been used to model clock delays around PLDs in order to reduce clock skew problems. Specifically, delays may be added to clock nets that have shorter clock paths so that they will match the delays experienced by the larger clock nets. This approach has been used to equalize the clock delays in such earlier PLDs.
Phase lock loop (PLL) circuits have been added to PLDs to generate clock signals with minimal clock skew and delay problems and improved setup and clock-to-output times. For instance, a PLL circuit may be coupled to receive an external reference clock signal applied to a clock pad of the PLD and to produce a duplicate version of the reference clock signal (same frequency) which is earlier in phase relative to the reference clock signal. This has been achieved by tapping a delay element in the feedback path of the PLL circuit so as to provide a PLL clock signal which is an early version of the external reference clock signal.
PLDs have been implemented in which a PLL has such a delay network in its feedback path that models the PLD clock tree so as to track over process, temperature and voltage to provide a consistent clock skew with respect to the reference clock. Such a delay network causes the PLL to generate an early clock that is ahead of the reference clock by an amount that compensates for the delay of the clock network. In this manner, chip clock skew can be canceled, and local clock drivers that propagate the PLL early clock can have relatively little clock skew with respect to the reference clock signal.
The drawing of FIG. 1 is a generalized schematic block diagram which shows a clock distribution network 20 employed in an earlier FLEX 10K programmable logic device which is presented merely as an illustrative example of the related art. Details of the FLEX 10K PLD can be found in the Altera 1996 Data Book produced by the Altera Corporation of San Jose, Calif. The clock distribution network of the PLD includes a clock pad 20 which receives an external clock signal. The reference clock signal is distributed about the device by a reference clock conductor path 24. The clock distribution network also includes a phase lock loop 26 which receives the reference clock signal via a driver circuit 28 and has a clock network delay compensation circuit in its feedback path which is tapped to produce an early (or leading) version of the reference clock signal which shall be referred to as the early PLL clock signal. The early PLL clock signal is distributed about the device by a PLL clock conductor path 30. The clock distribution network is divided into six localized sections. Four sections 32-1, 32-2, 32-3 and 32-4 provide clock signals to registers in the PLD periphery which latch data on external input/output pads (not shown). Two sections 34-1 and 34-2 provide clock signals to registers in the PLD core (details not shown) which is programmable to build logic functions. The clock network is constructed so that the delays remain substantially constant regardless of the manner in which the periphery and the core are programmed.
Each of the respective six sections of the clock distribution network 20 shown in FIG. 1 includes a respective clock selection circuit 38 which receives as inputs the reference clock signal and the early PLL clock signal. The reference clock signal is conducted along the reference clock conductor path and is provided to respective selection circuits via respective driver circuits. The early PLL clock signal is conducted along the early PLL clock conductor path 24 and is provided to respective selection circuits via respective local delay elements 40 which may be programmable.
The local delay elements, which are part of the clock distribution network, are balanced so that all of the respective selection circuits receive PLL clock signals that are in phase with each other and that are in phase with the reference clock signal received on the external pad. A larger local delay d1 is produced by the delay local elements 40 that are closer to the reference clock pad and the PLL since the distance traveled by the early PLL clock signal is shorter. Conversely, a shorter local delay d3 is produced by the local delay elements 44 that are farther from the reference clock pad and the PLL since the distance traveled by the early PLL clock signal is longer. Thus, the local delays impart different delay amounts at different locations in the clock signal distribution network so as to balance out or compensate for differences in clock signal delay experienced at different portions of the PLD. As a result, the early PLL clock signal is received at substantially the same phase by all six clock selection circuits.
While earlier clock signal distribution networks in PLDs generally have been acceptable, there have been problems with their use. For example, there may be situations in which a PLD is programmed so that registers that temporarily store logic signals are separated by long line delays or by delays due to combinatorial logic. More specifically, in a PLD programmed for synchronous operation, the total register-to-register delay is approximately the sum of register clock-to-output time plus line delay between registers plus register setup time. The PLD clock generally cannot be run faster than the register-to-register delay. Thus, when logic signals in a PLD must be conducted along relatively long paths between registers or along paths between registers that are delayed by combinatorial logic, then the register-to-register delay may have a significant impact on device performance.
Thus, there exists a need for a programmable logic device with a clock signal network that can more effectively manage register-to-register delays experienced by logic signals. The present invention meets this need.